The invention relates to apparatus for the purpose of preventing back flow of fluid, e.g. water, from a delivery line into a supply line, and more particularly to an improved valve of specifically advantageous modular-type construction, which may include a safety venting or "reduced pressure" feature.
Various types of apparatus have been used in connection with fluid supply systems for the purpose of preventing objectionable contamination due to back flow of contaminated fluid in a delivery piping to the supply piping. A simple check valve fulfills such a function, but there is always some danger of leakage sufficient to cause serious contamination, due, for example, to wear or failure of the valve or an obstruction which prevents proper closure. Therefore, more elaborate back flow preventer valve assemblies have been suggested, often including several check valves positioned in series fashion between the inlet and outlet ends of the assembly, thus providing a double safety feature. In addition, some of these assemblies have included a third safety feature comprising a venting apparatus connected between the two check valves to vent the space between these valves to atmosphere in the event of a leakage back through the downstream check valve. As an example of this type assembly, see U.S. Pat. No. 2,586,942.
Some types of assembly have utilized venting valves with fluid connections to both the space between the check valves and the fluid supply line, the valve being operable to vent the space between the check valves to atomsphere when the pressure in the space obtained a value substantially equal to the inlet pressure. With such apparatus it was assumed that no contamination of the fluid inlet line can occur because the pressure upon the outlet side of the first check valve can in no event exceed the inlet line pressure. However, this creates a danger of back flow leakage through the first check valve in the event of any malfunction with that valve, since pressure in the space between the two check valves is allowed to build up substantially to inlet pressure before any venting occurs.
The above cited U.S. Pat. No. 2,586,942 suggested one solution to this problem, utilizing a rather complex assembly including thin-walled diaphragm-like valve closure members designed to transmit pressure differentials in an upstream direction through the closed valves.
None of the previously suggested back flow preventer valve assemblies has provided a relatively simple, efficient and fail-safe back flow preventer valve as does the present invention described below.